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Concrete Calculator

A concrete calculator for slabs, footings and round columns: enter the size in feet and inches or metres and centimetres and get cubic yards, cubic metres and the number of premix bags, with a waste allowance added.

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Runs entirely in your browser. What you enter stays on your device — nothing is sent to a server. Verify it yourself

How do you work out how much concrete you need?

Multiply length × width × depth in feet to get cubic feet, divide by 27 for cubic yards, and add about 10% for waste. A 10 × 10 ft slab 4 inches deep is 33.3 cubic feet, or 1.23 cubic yards — 1.36 cubic yards with 10% extra.

Cost
Free, no limits
Upload
None — runs on your device
Sign-up
Not required
Works offline
Yes, once loaded

How to work out how much concrete you need

  1. Choose the shape

    Slab or footing for anything rectangular, Round column for posts and piers, or Area × depth if you already know the square footage.

  2. Choose the units and enter the sizes

    Feet and inches takes length and width in feet and depth in inches; Metres and cm takes metres and centimetres. Add the number of identical pours or columns.

  3. Set the waste allowance

    It starts at 10%. Raise it for uneven ground or awkward formwork.

  4. Read the volume and bag counts

    The volume to order is shown in cubic yards, cubic metres and cubic feet, with the approximate number of 80, 60 and 40 lb and 25 and 20 kg bags.

Examples

  • Slab 10 ft × 10 ft × 4 in, 10% extra1.358 yd³ (36.67 ft³, 1.038 m³), about 62 × 80 lb bags

    1.235 yd³ before the allowance.

  • Footing 20 ft × 2 ft × 12 in, 10% extra1.63 yd³ (44 ft³)

    A footing is the same box calculation as a slab.

  • Column 12 in diameter × 4 ft, 10% extra3.456 ft³ (0.128 yd³), about 6 × 80 lb bags

    π × 0.5² × 4 = 3.14 ft³ before the allowance.

Key features of the Concrete Calculator

  • Slab, footing, round column and area × depth shapes
  • Feet and inches or metres and centimetres
  • Results in cubic yards, cubic feet and cubic metres
  • Waste allowance, 10% by default, shown with and without
  • Approximate bag counts for 80, 60 and 40 lb and 25 and 20 kg premix
  • Quantity for several identical footings or columns

What it calculates

Shapes

  • Slab
  • Footing
  • Round column
  • Area × depth

Input units

  • Feet and inches
  • Metres and centimetres
  • Square feet or square metres

Results

  • Cubic yards
  • Cubic feet
  • Cubic metres
  • 80, 60 and 40 lb bags
  • 25 and 20 kg bags

When should you use this?

  • Ordering ready-mix for a patio, shed base or driveway slab.
  • Counting premix bags for fence-post holes or deck footings.
  • Converting a known area in square feet to cubic yards at a given depth.

Limitations

  • It assumes flat, even depths and straight sides. Sloping sites, thickened slab edges and stepped footings need to be split into parts and added up.
  • Bag yields are typical label figures, and the metric bag counts are estimated from the US yield per kilogram. They are approximate.
  • It does not account for reinforcement, hollow blocks or anything else that displaces concrete inside the form.

Troubleshooting

The result is far larger than expected

Check the depth unit: it is inches (imperial) or centimetres (metric), not feet or metres.

No result appears

Every dimension must be above zero and the quantity a whole number of 1 or more. A message says which input is the problem.

Common mistakes to avoid

  • Entering the depth in feet when the field asks for inches. 4 means 4 inches here; 0.33 would mean a third of an inch.
  • Ordering exactly the calculated volume. Sub-base is never perfectly level, so allow extra.
  • Using the diameter as the radius for a column, which quadruples the volume.

Pro tips

Tips for better results

  • Measuring

    Average the depth across the compacted sub-base

    Measure the depth in several places after the sub-base is compacted and use the average — a slab that is half an inch deeper than planned uses about 12% more concrete at 4 inches.

  • Odd shapes

    Split irregular areas into rectangles and add them

    For an irregular area, split it into rectangles, work each out with the same depth and add the volumes.

  • Ordering

    Past a cubic yard, price ready-mix against bags

    Bag counts climb fast: above about one cubic yard, compare the price of ready-mix delivery with 45 or more 80 lb bags.

How to convert square feet to cubic yards

Square feet is an area and cubic yards is a volume, so you need the depth as well. Multiply the area in square feet by the depth in feet (inches ÷ 12) to get cubic feet, then divide by 27, the number of cubic feet in a cubic yard. For square feet to cubic yards for concrete at the common 4-inch slab depth, that is square feet × 0.333 ÷ 27, or square feet ÷ 81: 81 sq ft at 4 inches is exactly 1 cubic yard, and 1,000 sq ft is 12.35 cubic yards.

Square feet to cubic feet is the first half of the same sum: square feet × depth in feet. A 100 sq ft slab 4 inches deep is 33.3 cubic feet. The Area × depth tab does this if you already know the area.

Slabs, footings and columns

A slab or a strip footing is a box: length × width × depth. A round column or post hole is a cylinder: π × radius² × height. The calculator works each shape out in cubic metres, multiplies by the number of pours or columns, adds the waste percentage and shows the result in cubic yards, cubic feet and cubic metres.

Bags of premix

US premix bags are commonly labelled to yield about 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. The calculator divides the volume to order by those yields and rounds up. Metric bags are not labelled consistently, so the 25 kg and 20 kg counts use the same yield per kilogram as the 80 lb bag — about 11.7 and 9.4 litres a bag. Treat every bag count as approximate and check the yield printed on the bag you buy.

Sources

Privacy

The Concrete Calculator runs entirely in your browser. What you enter and the result it produces stay on your own device — nothing is sent anywhere, because nothing needs to be.

You can confirm this rather than take our word for it: open your browser’s developer tools, watch the Network tab, and use the tool. No request carries your input. It also keeps working with the connection off.

Details are in the privacy policy.

More calculators

What next?

Convert the cubic yards of sub-base to tons.

Weigh a load of gravel or sand

Concrete Calculator — questions

How many cubic yards of concrete do I need for a 10 × 10 slab?

At 4 inches deep, 10 × 10 ft is 33.3 cubic feet, or 1.23 cubic yards. With 10% extra for waste that is 1.36 cubic yards (36.7 cubic feet), or about 62 bags of 80 lb premix.

How do you convert square feet to cubic yards?

Multiply square feet by the depth in feet, then divide by 27. At 4 inches (0.333 ft) deep, that is the same as dividing square feet by 81; at 6 inches, by 54.

How much extra concrete should I order?

The calculator defaults to 10%, a common allowance for uneven sub-base, formwork and spillage. You can set anything from 0 to 100%. Running short in the middle of a pour is worse than having a little left over.

How many bags of concrete make a cubic yard?

About 45 bags of 80 lb (27 ÷ 0.60), 60 bags of 60 lb or 90 bags of 40 lb, using the yields typically printed on US premix bags. Brands and mixes differ, so check the bag.

Does it work in metres?

Yes. Choose Metres and cm: length and width (or height) are in metres, and depth and diameter in centimetres. The results are still shown in both cubic metres and cubic yards.

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